Currently commercialized article surveillance systems are generally of the low frequency magnetic type, radio frequency type or microwave electromagnetic/electrostatic type. In the case of an expanded width control zone the installation choice has typically been based on either radio frequency or microwave transmission or combinations of both.
Where a control zone is of relatively narrow width, such as the checkout area for a supermarket or similar retail establishment, a magnetic system would be preferable, at least in part because magnetic systems tend to be less expensive and because magnetic systems are more effective and efficient with the type of products in a supermarket. However, magnetic systems are more likely to be adversely affected by metallic fixtures, metal counters, metal shopping carts, noise generators such as laser product scanners and the like. Such nearby conditions may create signals of their own which interfere with the effective and efficient operation of a magnetic theft deterrent system.
In some installations where magnetic systems have been installed, it has been necessary to remove all nearby metal fixtures to minimize the possibility of such interference. Also the presence of a magnetic tag adjacent to but outside the control zone may result in a false alarm signal. Thus, magnetic systems, despite their many other advantages, have serious disadvantages in supermarket and like retail store operations. It would be desirable to have a magnetic system where such metal and ambient noise generators, although present, do not interfere with detection of the actual theft deterrent tags in the control zone.
In addition, since the aisle width of a supermarket through which the customer must pass is relatively narrow, it is advantageous that the article surveillance equipment, including transmitting and receiving antennas take up as little width space as possible in order not to impinge on normal passage through the aisle.